In die-casting of metal material (e.g., aluminum), it is necessary to reduce an abrasion/friction of the sleeve on sliding surfaces between the plunger for injecting molten metal into the die-cast mold and the sleeve in which the plunger reciprocates, and to stabilize an injection speed by the plunger. Therefore, a plunger lubricant is supplied between the plunger and the sleeve (especially, onto inner walls of the sleeve). As the lubricant, an oil-based lubricant is widely used, the oil-based lubricant being an oil in which solid substances, such as graphite powders and talcum powders, are dispersed. Such an oil-based lubricant in which the solid substances are dispersed has an excellent lubricating property.
However, when the oil-based lubricant in which the solid substances are dispersed is used, residues (especially, black residues) adhere to a surround/circumference of the die-cast apparatus. The residues are hard to be cleaned up because they contain the oil, the graphite, or the like, in large amount, and thus, there arises a problem of worsening a working environment. In addition, when the molten metal is poured into the sleeve after the oil-based lubricant is provided into the sleeve, the oil-based lubricant is exposed to a high temperature environment, and thus, the oil components may burn. This may damage the plunger, the sleeve, and the like, of the die-cast apparatus. Further, gases generate due to burning of the oil components, and when the gases mix into the molten metal, internal defects (e.g., casting porosity) occur in the cast product, resulting in a deterioration in quality of the cast product.
In order to solve the above described problems caused by the oil-based lubricant, a lubricant having low flammability is required. As such a lubricant, a water-soluble lubricant in which water is added to the oil-based lubricant is proposed. Patent literature 1 discloses a lubricant which is a W/O type emulsion containing a mineral oil, a grease, or the like, used as a base oil, and a surface-active agent. Patent literature 2 discloses a water-soluble lubricant in which a metal salt of higher fatty acid serving as a base compound is dispersed with a surface-active agent.